Many of the shopping engines have complex categorization logic built into their feed processing systems. Even though SingleFeed provides the correct category for a product, an engine may override SingleFeed's category with their own, by using logic to determine a category they think is more appropriate. However, these systems are far from perfect and you may frequently find that the shopping engines have miscategorized your products.
This issue is especially apparent on Shopping.com, where they frequently put more weight on your product name, description, UPC and MPN than on the category that Singlefeed provides. A few shopping engines have similar yet less frequent issues including Shopzilla, Amazon, The Find, and Yahoo Shopping.
The first step to correcting these issues is to ensure that you are including valid identifiers (MPN, UPC) in your SingleFeed. If that doesn't work, review your product names and descriptions. You'll often find that the shopping engine latches onto a keyword that leads them to an incorrect conclusion about the product category. For example, a "Gel iPhone Skin" may get categorized as Health and Beauty due to the keywords "skin" and "gel".
Modifying your product information isn't always desirable, so many engines allow you to override their auto-categorization by manually setting new categories within their account center. Shopping.com even has a way for you to submit a remapping sheet where you can specify mappings between SingleFeed's categories and theirs.
Finally, you may contact the shopping engine directly in order rectify the problem.
A further note on Shopping.com: Often times they will have products categorized correctly - based on the categories SingleFeed provides - yet they display "May Need Review" in the account center. This means that, although they are using the correct categories, their processing system feels uncertain about the accuracy of that categorization. This is usually due to missing or incorrect identifiers (MPN, UPC), and/or a mis-match with how your product is categorized versus the majority of merchants selling that same product. If you find that the categories are accurate, there is no need to take any action, but know that they will continue to display "May Need Review" in the Shopping.com account center.